As a fuel cell system to be mounted on a vehicle, a system comprising a plurality of DC-DC converters has been developed. For example, JP-A-2007-209161 discloses a fuel cell system comprising a first DC-DC converter disposed between an electric storage device and an inverter, and a second DC-DC converter disposed between a fuel cell and an inverter.
With this system, when the target motor output is greater than a predetermined threshold, the driving of the first DC-DC converter is stopped, the second DC-DC converter is set to an electrically, directly-connected state, and the output power of the fuel cell with greater output is preferentially supplied to the motor over the output power of the electric storage device. Moreover, when the target motor output is smaller than the threshold, the first DC-DC converter is operated to supply assisted power from the electric storage device, and the second DC-DC converter is set to an electrically, directly-connected state. According to this kind of configuration, it is possible to prevent the traveling performance of the vehicle from deteriorating, and enable efficient power conversion (Patent Literature 1).